Auto manufacturers are looking for a rugged, long life, and efficient light source to replace tungsten filament headlamps. Automobiles are harsh environments for a light source. While a vehicle may have a life of ten years, current light sources have lives substantially less than this. Ideally the headlamp should last as long as the motor. If a motor is rated at a life of ten years, a light source should then be capable of roughly 5000 lamp starts, and 5000 hours of lamp operation. Typical tungsten halogen lamp sources in use today are capable of about 1000 starts and 2000 hours of operation. Not only should a lamp not fail abruptly, a lamp's quality should not degrade over time. An automobile light should maintain its level of light output over its operative life. Tungsten halogen lamps currently in use slowly evaporate the tungsten filament. The tungsten is then deposited on the reflector and lens, thereby darkening them and reducing the total useful light output. There is then a need for an automobile headlight capable of a life comparable to the life of a vehicle, for example about 5000 starts, and 5000 hours operation, without loosing much of its initial output, for example less than about 15% of its light output over the life of the lamp.
Automobile headlights are necessarily positioned along the front surfaces of the vehicle. These surfaces are exactly the surfaces that first encounter wind resistance as the vehicle moves. Lamp faces are therefore important to the aerodynamic design of a vehicle. While large lamp faces may be sculpted to conform to a particular aerodynamic design, the economic benefit of mass producing a standardized lamp is then lost. There is a then need to limit the size of lamps to have as little wind resistance as possible. There is a corresponding need to limit lamp size, so as to encourage headlamp standardization.
To make headlamps as small as possible, and as inexpensive as possible, plastic is used for lenses and reflectors, since plastic is both inexpensive and may be precisely molded. The use of plastic and the need for compact headlamps creates a possible problem with over heating. It is possible to melt plastic. It is thus desirable to put as few watts as possible into the assembly, using the energy as efficiently as possible. There is then a need for a headlamp that produces an adequate amount of light with the least amount of energy, and the greatest efficiency.
Electroded HID lamps are commonly produced by press sealing a glass envelope around the electrodes. While the unmelted portions of the envelope may be accurately controlled in manufacture, the wall thicknesses, and wall angles of the press seal are variable. A small but still significant portion of the lamp light passes through or is reflected from the press seal, particularly in smaller or shorter lamps where the seal area is a greater portion of the sphere of illumination. The variable wall features of the press seal cause uncontrolled deflections of light that result in glare. There is then a need for an HID lamp that has accurately controlled wall thicknesses, and wall angles.
Optical path designs could be made ideal in three dimensions, if there were ideal point sources of light. Similarly, display systems could be made ideal in two dimensions if there were ideal linear light sources. Unfortunately, there are no ideal point or linear light sources. As a result, the lighting paths designed in reflector, and lens systems are complex compromises. The compromises are manifested in larger, more complex and more expensive reflectors and lenses, but size and complexity are in conflict with aerodynamics and cost. There is then a need to produce a more nearly ideal point or linear light source to enable simplification of reflectors and lens, or improve the quality of output beams.
Conventional, large size electroded arc lamps can have efficiencies of 80 lumens per watt. The electrode heat losses are a small fraction of the energy input to the lamp, for example a 20 watt loss for a 400 watt lamp. When the lamp size is reduced to a size appropriate for an automobile, for example where the total power input is only about 20 watts, the electrode losses dominate and present a formidable energy budget problem. There is then a need for an energy efficient, small arc discharge lamp.
For high wattages, HID lamps are efficient light sources producing approximately 80 lumens per watt. Unfortunately, at low wattages of about 10 or 20 watts, or less, normal electroded type HID lamps do not operate efficiently. Most of the energy is dissipated in heating the electrodes, and the surrounding envelope material. At higher wattages, for example more than 30 watts, where electroded HID lamps operate more efficiently, more light is produced than desirable for automotive headlights. The light source is also generally larger than convenient with regard to coupling to headlamp reflector optics. The light output of an automobile headlight must be controlled, both as to total lumens, and direction. Excess light may be absorbed, possibly resulting in harmful heating of the absorber. Excess light may also be deflected; but deflected light may result in glare for other drivers, or even though deflected from the beam, may be reflected back to the driver in veiling glare, especially in rain, fog or snow. Excess light is then a problem, and current forms of electroded HID lamps may be regarded as being too powerful for automobiles. There is then a need for an HID lamp that efficiently produces about 2000 to 3000 lumens in the region of 20 to 30 watts.
Examples of the prior art are shown in U.S. Pat. Nos. 3,763,392; 4,812,702; 4,002,943; 4,002,944; 4,002,944; 4,041,352; 4,887,008; and 4,887,192.
U.S. Pat. No. 3,763,392 Hollister broadly shows a light transmissive sphere containing a high pressure gas that is induced to radiate by an induction coil surrounding the sphere.
U.S. Pat. No. 4,812,702 Anderson discloses a toroidal coil for inducing a toroidal discharge in a containment vessel. Anderson emphasizes the use of a V shaped torus cross section.
U.S. Pat. No. 4,002,943 Regan shows an electrodeless lamp with an adjustable microwave cavity. The cavity is designed to be expandable or contractible by threading two wall portions together.
U.S. Pat. No. 4,002,944 McNeill discloses an electrodeless lamp using a resonant cavity to contain the lamp capsule. A tuning element is inserted in the cavity to adjust the cavity resonance.
U.S. Pat. No. 4,041,352 McNeill shows an electrodeless lamp with an included capacitor to assist in lamp starting. On ignition, a switch disconnects the capacitor, allowing full power to flow to the discharge gas.
U.S. Pat. No. 4,887,008 Wood shows an electrodeless lamp in a microwave chamber shielded with a light transmissive mesh opaque to microwave energy.
U.S. Pat. No. 4,887,192 Simpson shows an electrodeless lamp with a well defined, metallic compound resonant cavity.